We demonstrate that a reversible semimetallic-to-metallic transition can be realized in monolayer graphene by iodine doping and dedoping processes. Upon iodine doping, the charge transfer from graphene to iodine creates a high hole density up to 4.75 × 10(13)cm(-2), well beyond that realized by applying gate voltages. Iodine-doped graphene shows metallic behaviour, as evidenced by the resistance variation with temperature and magnetic field. We further introduce an iodine dedoping method to completely remove the iodine anions from graphene surfaces. Transport measurements show that after dedoping treatments, the mobility of graphene is significantly enhanced, much higher than that of pristine graphene. The improvement in graphene electronic properties is attributed to the corrosive characteristic of iodine that it can react with and remove absorbed atoms on graphene surfaces. Our work not only opens a facile and effective way to tune the properties of monolayer graphene reversibly, but also demonstrates a new method to increase the quality of graphene.